Optical communications systems utilizing lasers, and having high-bit rate modulation and/or wavelength division multiplexing, require accurate control of laser wavelength. Such accurate control is difficult to implement in present transmission systems because the laser wavelength is monitored by using elaborate and complex optical components and/or circuit elements. Some examples of the prior art include: U.S. Pat. Nos. 3,588,253, 4,387,462, and Japanese Patent Disclosure No. 1983-77272.
U.S. Pat. No. 3,588,253 discloses a spectrograph which includes a temperature tunable laser diode. The laser diode is placed in a Dewar flask and liquid helium is used to control the temperature of the environment in which the laser diode is positioned. Since the wavelength of single mode emission from the laser diode depends upon the temperature thereof, the wavelength of the single mode emission from the diode is varied by varying the temperature of the diode.
U.S. Pat. No. 4,387,462 discloses a temperature stabilization apparatus which stabilizes a single frequency laser. The laser is disposed in a hard-sill glass laser tube having a resistance coating on an exterior surface thereof. A power supply is coupled to the resistance coating for heating the glass tube. Temperature sensors are mounted at various locations to monitor the temperature of the laser tube. The temperature sensors are coupled to a temperature comtroller, which in turn, is connected to the power supply for controlling the temperature of the tube.
Japanese Pat. No. 1983-77272 discloses an apparatus for temperature control of a semiconductor laser. The apparatus uses a photodiode, having a predetermined voltage-temperature characteristic, to sense the temperature of a heat sink upon which the photodiode and the laser are disposed. A thermo-module adjusts the temperature of the laser in response to variations in the photodiode output voltage.
If the large bandwidthe capability of presently available single-mode fibers is to be used to its full capacity, it is necessary to develop a compact and integrated wavelength stabilization apparatus for laser transmitters other than those disclosed in the art.